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strings, file paths, and web service URLs. Because the configuration file can be modified at any time, this allows you to update the configuration of an application as its physical deployment characteristics change without needing to recompile it. Custom settings are entered using an <add> element that identifies a unique variable name (the key) and the variable contents (the value). The following example adds two new custom configuration settings: < xml version="1.0" > <configuration> <appSettings> <add key="websiteName" value="My New Website"/> <add key="welcomeMessage" value="Welcome to my new Website, friend!"/> </appSettings> <system.web>...</system.web> </configuration> Once you ve added this information, .NET makes it extremely easy to retrieve it in your webpage code. You simply need to use the ConfigurationSettings class from the System.Configuration namespace. It exposes a property called AppSettings, which contains a dynamically built collection of available application settings for the current directory. For example, if the ASP.NET page class referencing the AppSettings collection is at a location such as http://localhost/MyApp/MyDirectory/ MySubDirectory, it is possible that the AppSettings collection contains settings from three different web.config files. The AppSettings collection makes that hierarchy seamless to the page that s using it. To use the ConfigurationSettings class, it helps to first import the System.Configuration namespace so you can refer to the class without needing to use the long fully qualified name, as shown here: using System.Configuration; Next, you simply need to retrieve the value by name. The following example fills two labels using the custom application information: protected void Page_Load(object sender, EventArgs e) { lblSiteName.Text = ConfigurationManager.AppSettings["websiteName"]; lblWelcome.Text = ConfigurationManager.AppSettings["welcomeMessage"]; } Figure 5-4 shows the test web page in action.

The previous example shows how you can handle application events in a custom HTTP module. However, some global events aren t provided by the HttpApplication class but are still quite important. These include events raised by other HTTP modules, such as the events fired to start and end a session. Fortunately, you can wire up to these events in the Init() event; you just need a slightly different approach. The HttpApplication class provides a collection of all the modules that are a part of the current HTTP pipeline through the Modules collection. You can retrieve a module by name and then use delegate code to connect an event handler. For example, if you want to connect an event handler named OnSessionStart() to the SessionStateModule.Start event, you could use code like this for the Init() method in your HTTP module: Public Sub Init(ByVal httpApp As HttpApplication) Dim sessionMod As SessionStateModule = httpApp.Modules("Session") AddHandler sessionMod.Start, AddressOf OnSessionStart End Sub

To turn off view state for a single control, set the EnableViewState property of the control to false. To turn off view state for an entire page and all its controls, set the EnableViewState property of the page to false, or use the EnableViewState attribute in the Page directive, as shown here: <%@ Page Language="c#" EnableViewState="false" ... %>

An error won t occur if you try to retrieve a value that doesn t exist. If you suspect this could be a problem, make sure to test for a null reference before retrieving a value.

Summary

Even when you disable view state for the entire page, you ll still see the hidden view state tag with a small amount of information. That s because ASP.NET always stores the control hierarchy for the page at a minimum, even if view state is disabled. There s no way to remove this last little fragment of data.

Values in the <appSettings> element of a configuration file are available to any class in your application or to any component that your application uses, whether it s a web form class, a business logic class, a data access class, or something else. In all these cases, you use the ConfigurationSettings class in the same way.

In this chapter, you took a closer look at what constitutes an ASP.NET application. After learning more about the life cycle of an application, you learned how to code global application event handlers with the global.asax file and how to set application configuration with the web.config file. Finally, you learned how to use separately compiled components in your web pages and how to extend the HTTP pipeline with your own handlers and modules.